This invention relates to industrial controls such as overload relays and motor controllers and in particular, to a pilot light for such industrial controls that serves to provide both a visual indication of the operation of the control and to provide machine readable data imperceptible to the human eye. More generally, the invention relates to a low cost digital, optical interface for small electronic apparatuses.
Industrial controls, such as overload relays and motor controllers, are elemental components of industrial electronic control systems such as are found in office buildings and factories. An overload relay, for example, is used with a contactor to control electrical power to large motors. The contactor incorporates one or more contact sets which switch power to the motor's windings. The contact sets are closed by movement of an armature attracted by a magnetic field generated by an armature coil. Thus, the contactor allows the relatively high currents of the motor winding to be controlled by a lower current through its armature coil.
The overload relay receives control signals from remote motor start and stop buttons, and switches the current in the armature coil of the contactor in response to those signals. Importantly, the overload relay also monitors the current flowing through the contact sets and interrupts current to the contactor's armature if the power dissipated by the motor increases beyond a predetermined limit. Increases in motor current may be caused by structural failure of the motor, such as a short between its windings, or by a loading of the motor beyond its rated capacity.
The overload relay may sense the current flowing through the contact sets by means of a resistive heater connected in series with one or more of the contact sets. The resistive heater is in thermal contact with a bi-metallic strip, which at high currents deflects to open a switch and stop current flow to the contactor's armature coil. This, in turn opens the contact set disconnecting the motor windings from the power.
More recently, the current sensing in overload relays has been accomplished by current transformers connected in series with each of the contact sets of the contactor. The current transformers generate an electrical signal, which may be compared to a reference signal to actuate a solid state device, such as a transistor for controlling the armature current.
With the decreasing cost of microprocessors, it has become feasible to incorporate a microprocessor into the overload relay to perform the functions previously performed by discrete electronic circuitry or a bi-metallic strip. A further attraction to the use of a microprocessor is the possibility of performing additional functions, beyond overload protection, such as monitoring and storing data related to the operation of the motor. Although using a microprocessor to collect and store data is relatively straightforward, in the context of the small package size and low cost of the overload relay, communicating the data collected to a human operator is a problem.
A general purpose alphanumeric display is impractical both for reasons of size and cost. A display large enough to be easily readable requires an unworkably large mounting area. Support circuitry and protection from the industrial environment further increase the space demanded by such displays. Increasing the size of the overload relay is unacceptable to the extent that it adversely affects the use of the overload relay in high density wiring applications. The present cost of such displays would significantly increase the cost of the overload relay.
As an alternative to an on-board display, an electrical jack might be used for connection of each overload relay to a remote display device. This avoids some of the cost and space problems described above which requires the user of the overload relay to purchase one or more specialty displays to ensure that a display may be available at the site of each overload relay. Precautions must be taken to prevent contamination of the jack in the industrial environment.